Research On Key Fundamental Technology Of Digital Oral Implantology

The application of digital technology in oral implantology has changed the concept abouttraditional prothsodontics radically and enabled the proe and post surgical simulation to be achieved,which promotes oral implanting theory to develop from traditonal qualitative to quantitative status,and starts a new technological revolution in oral implantology. By combining the practical applicationof oral implantology and various interdisciplinary theories and methods, this dissertation launches theresearches on teeth segmentation, gingival contour extraction and reconstruction, implant surgicalplanning, and custom abutment design in order to improve the successful rate and aesthetic effect. Themain research works and results are as follows:(1) For the issues of the fuzziness of CT slice images and the variability of tooth topologicalstructure, a novel teeth segmentation method based on a mixed level set active contour modelcombined edge and local area is presented. According to the mapping mechamism between layers,different models are adopted on different slices: the single mixed level set model for dental root layerslices and the double model for dental crown layer slices. Meanwhile, the narrowband method, theprior shape restraint, and the gradient direction restraint are introduced to overcome the disturbance ofthe jawbone. Aimed at the teeth adhesion phenomenon in the dental crown layer slices, regionalcompetitive principle is also added in order to extract the common border between the adjacent teeth.The method can greately deal with splitting and merging of teeth topology structure and therefore caneffectively extract each tooth profile without extra supervision and intervention during thesegmentation process.(2) Aimed at the problem of the adjacent teeth adhesion of dental meshes, a single-tooth gingivalcontour extraction method is proposed. According to the anatomical characteristic of dental model, anapproach combining the discrete curvature analysis and the shortest path search is applied to obtainthe gingiva feature line.The gingival data points of each tooth are then segmented accurately by usingthe B-spline curve fitting based on the energy optimization and by the differential properties of thefeature line. For restoring the missing gingival shape between teeth, the B-spline curve extendedfitting to the dental arch, the curve intersection by knot insertion, and the morphology modeling of thetangent contraints between teeth are adopted to gain the closed gingival curve of each tooth with twoorder continuous characteristic and no interference between them. The experiments show that thepresented method is effective not only for the aligned dental model, but also for the misaligned andthe model with missing teeth. (3) For restoring the ideal aesthetic gingiva controur for edentulous patients, a gingivalreconstruction method based on statistical model is proposed. Introducing the technology of thegingival contour extraction, an automatic calibration of gingival shape according to the teethanatomical characteristics is presented. Through principal component analysis, the statistical model ofgingival contours is built from a3-D scan dental dataset. Within this statistical model, themorphologically plausible missing contours can be inferred based on a nonlinear optimization fittingfrom the statistical deformation parameters adjustion, the Mahalanobis prior, and the global similaritytransformation, as well as the elastic deformation based on the radial basis function.(4) On the basis of taking into account the factors about prosthodontics, aesthetics, bone qualityand quantity, a parametrized planning method for preoperative dental implant positioning is presented.Through the gingival contours the implanting region is first restricted. By computing the bone densitycenter in this region of the CT images and searching the radial bone mass on the cutting plane, theappropriate implant positioning is determined. Based on the image sampling and interpolation theradial safety distance of the bone tissue around implant and the bone quality and quantity parametersare evaluated to guarantee the stability of the implant-bone interface. According to the positioningplanning results a parametric mesh modeling method for the positioning structure of surgical guide ispresented, which can fast and accurately transfer the implant positioning information to the oral cavityof patients. The method can offer the optimal surgical planning for dentists, effectively improve thepast inaccurate problem from dentist experience and hand-made design for surgical guide, and raisethe success rate of operation.(5) For improving the gingival shape after implant restoration, a digital design method for customabutment is proposed based on the suface extraction and gingiva reconstruction technology. Throughobtaining the patient dental model with healing abutment and recognitioning its surface characteristicsbased on the geometric constraint, the position of the abutment and implant in the alveolar bone isobtained quickly and accurately. To correct the inclination of the implant, a method for computingtooth long axis based on the tooth anatomical characteristics is presented to acquire the oblique angleof the abutment. To make the custom abutment has aesthetic anatomical characteristics, the gingivalreconstruction technique is applied to obtain the best matching anatomical gingival curve withpatients, which guides the design for the neck anatomical shape of custom abutment, and thereforesimplifis the design process. The method abandons the troublesome procedures of traditional waxingand impression transfer, and greatly improves the design efficiency and accuracy.